Linear motion axes are ubiquitous in manufacturing systems. For high speed or high accuracy systems, rolling element bearings or pressurized fluid bearings dominate. In the latter case, air bearings are often used. In order for air bearings to be used, however, they must be preloaded, which means that the carriage supporting the air bearings must wrap around the linear motion axis and the bearings must act on all sides of the axis structure. In addition, the axis structure must be machined with all sides extremely straight and parallel, or else varying distances between surfaces will cause the air bearing pads to touch down and fail.
Some designers try to overcome this by utilizing springs to preload one set of bearing pads; but, this adds to complexity. Others use vacuum pads to preload air bearings, thereby eliminating the need to wrap around the structure. Vacuum, however, can only generate an order of magnitude less force than can the air bearings themselves; and, therefore, vacuum preloaded air bearings systems are not good at resisting overturning moments. In addition, for vertical motion travel, if the vacuum is lost, the system can fall apart and crash catastrophically.
One of the earliest precision linear motion systems, which still enjoys widespread use, is the so-called Vee and Flat configuration. The rail, upon which the carriage rides, has a Vee-way protrusion and a Flat-way, as later discussed in connection with FIG. 1. The carriage has a mating Vee and Flat, and gravity preloads the system. The bearings for Vee and Flat systems can be sliding, rolling, hydrostatic, or aerostatic. In addition, linear electric motors with high attractive forces have been used. The principal problem with Vee and Flat systems, or double Vee systems for that matter, is that grinding the required precision angled surfaces is more complex and less accurate than grinding rectangular shapes. In addition, for the Vee and Flat system, there are three precision surfaces that must be created for the bearings. The generation of accurate rectangular shapes is well documented in W. Moore's Foundations of Mechanical Accuracy, published by Moore Tool Company, Bridgeport, Conn., which discusses how making a straight edge with right angle surfaces is a fundamental step, and how making Vees is a step that follows from it, such that rectangular shapes can fundementally be made more accurate.
Furthermore, with respect to accuracy, a Vee and Flat design has coupled motions. A horizontal force, resisted by the bearings on the Vee, causes deflections in the bearings that engage the Vee and this results in an error motion not only in the horizontal direction, but also in the vertical direction. Since the vertical error component only occurs over the Vee, the supported carriage also incurs a roll error motion. Double Vee way systems partially overcome this problem with the use of two Vees, but there is still coupled motion in the vertical direction. In addition, double Vee way systems are very difficult to manufacture to obtain parallelism between the Vees.
Many high-speed systems use linear electric motors because they provide a simple design with extremely high acceleration rates and high servo stiffness. Many permanent magnet motors, however, have a very high attractive force between the coils and the magnets. For example, if a motor has 1000 N of linear motion force, it may have 6000 N of attractive force. Until the advent of the present invention, indeed, such high motor forces acted only in one direction, thereby only preloading one set of air bearings, unless the more complex Vee and Flat arrangement is used, as discussed above; and the high attractive forces greatly decreased the life of rolling element linear motion bearings.
A significant manufacturing development towards attaining low cost aerostatic bearing systems was provided in U.S. Pat. No. 5,488,771, "Method for Manufacturing Externally Pressurized Bearing Assemblies", of common assignee herewith. In accordance with the teaching of this patent, the fluid bearings are suctioned to a bearing rail by vacuum, and then the carriage is positioned over the bearings and aligned. An epoxy is then injected between the bearings and the carriage. This same method is also useful in the implementation of the present invention, as later explained, with the significant addition, provided by the invention, of unique angular positioning of the linear electric motor with respect to the bearings, which ride on a very simple and accurate rectilinear bearing rail.